1. Field of the Invention
The present invention relates to an electronic device for enhancing voltage driving efficiency for a source driver and a liquid crystal display (LCD) monitor thereof, and more particularly to an electronic device and a LCD monitor thereof for enhancing efficiency of driving grayscale reference voltages to reduce total charging time for equivalent capacitance units.
2. Description of the Prior Art
A liquid crystal display (LCD) monitor featuring slim design, low power consumption, and no radiation pollution has been applied widely to a computer system, a mobile phone, a Personal Digital Assistant (PDA) and so on. The operation principle of a LCD monitor is based on different alignments of liquid crystal molecules with different effects of polarization and deflection. By means of different alignments of the liquid crystal molecules, the light can be allowed to pass through in varying amount, thus constituting different intensities of the emitting light and different levels of grayscales in red, blue and green.
Please refer to FIG. 1, which is a schematic diagram of a thin film transistor (TFT) LCD monitor 10 according to the prior art. The LCD monitor 10 includes an LCD panel 100, a timing control circuit 102, a data line output circuit 104, and a scan line output circuit 106. The LCD panel 100 includes two substrates with liquid crystal material in between. One substrate has a plurality of data lines 110, a plurality of scan lines (gate lines) 112 perpendicular to the data lines 110, and a plurality of TFTs 114. For convenient explanation, only four TFTs 114 are shown in FIG. 1. There exists one TFT 114 at every intersection of each of the plurality of data lines 110 and scan lines 112 in practice. In other words, the TFTs 114 are distributed on the LCD panel 100 in matrix. Each data line 110 corresponds to a column of the LCD monitor 100, each scan line 112 corresponds to a row of the LCD monitor 10, and each TFT 114 corresponds to a pixel. Furthermore, the circuit characteristic of the two substrates of the LCD monitor corresponding to each pixel is regarded as an equivalent capacitance unit 116.
In the LCD monitor 10, the timing control circuit 102 generates input signals to the data line output circuit 104 and the scan line output circuit 106, respectively. The scan line output circuit 106 inputs a pulse into the scan lines 112 to conduct the TFTs 114, and thereby voltage signals driven from the data line output circuit 104 to the data lines 110 can be transmitted to the equivalent capacitance units 116 through the TFTs 114 to control the gray level status of the corresponding pixel.
As the LCD monitor 10 is a large size monitor, the data line output circuit 104 usually includes multiple source drivers. Each source driver is responsible for signal output to data lines 110. The main function of the source drivers is to transfer the received digital grayscale data into analog driving voltages and perform Gamma correction, driving voltage polarity control, etc.
Please refer to FIG. 2, which is an internal schematic diagram of a source driver 20 of the data line output circuit 104 shown in FIG. 1. A LCD panel 26 coupled to the source driver 20 can display 64 levels of grayscales (denoted by values 0-63). The source driver includes a reference voltage generator 22, a data latch 24, digital to analog converters (DACs) DAC(1)-DAC(640), coupling lines L1-L64 and operational amplifiers (OPs) OP(1)-OP(64). The data latch 24 is utilized to receive 6 bits (capable of representing grayscale values from 0 to 63) grayscale data values GD0-GD63 which correspond to DAC (1)-DAC (640) and equivalent capacitances C1-C64 on the LCD panel 26 respectively. The reference voltage generator 22 generates grayscale reference voltages GV0-GV63 which are regarded as Gamma correction voltages when the source driver 20 equipped with the Gamma correction function. The OP (1)˜OP (64) drive the grayscale reference voltages GV0-GV63 to feed DAC (1)-DAC (640) through the coupling lines L1-L64, respectively. For simplicity, assume that the voltage gain of OP (1)˜OP (64) is equal to 1. Based on GD0˜GD639, DAC (1)˜DAC (640) select the corresponding coupling lines to output the corresponding reference voltages to the equivalent capacitances C1-C640. For example, if grayscale data values GD0 and GD1 are 32 and 63 respectively, the DAC (1) and DAC (2) selects the coupling lines L33 and L64 to output the grayscale reference voltages GV32 and GV63.
As mentioned above, when more DACs receive the same grayscale reference data, the coupling line corresponding to the grayscale reference data is selected by the DACs at the same time. This makes the corresponding OP's loading heavier such that it is hard to drive the grayscale reference voltages. More charging time will be needed for the equivalent capacitances, resulting in decrease of the displaying efficiency.